I had the same problem, I think since I last service the car in Sept '15. So spoke to Audi JB service personnel, and got appointment to send in the car yesterday.

Had the car back on the same day, the culprit was the spare battery (the one that assist the crank battery). Under warranty so FOC. It seems this will happen to all A6H so better pray the spare battery konk before the 4 years warranty end.